[cmake] Prefix gtest and gtest_main with "llvm_".

The upstream project ships CMake rules for building vanilla gtest/gmock which conflict with the names chosen by LLVM. Since LLVM's build rules here are quite specific to LLVM, prefixing them to avoid collision is the right thing (i.e. there does not appear to be a path to letting someone *replace* LLVM's googletest with one they bring, so co-existence should be the goal).

This allows LLVM to be included with testing enabled within projects that themselves have a dependency on an official gtest release.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D120789